.f {
    width: 400px;
    height: 400px;
    background-color: pink;

    &:hover {
        background-color: palevioletred;
    }

    .son {
        width: 200px;
        height: 200px;
        background-color: skyblue;

        // 鼠标进过son变色
        &:hover {
            background-color: greenyellow;
        }

        // 鼠标经过son 下面的第四个孙子变红
        &:hover .sun:nth-child(4) {
            color: lightcoral;
        }

        .sun {
            width: 30px;
            height: 30px;
            background-color: orange;
            color: rebeccapurple;

            &:hover {
                background-color: olivedrab;
            }
        }
    }

}